1. Field of the Invention
The present invention generally relates to position sensors used for sensing the absolute position of a steering wheel or other like device which normally undergoes more than a single revolution during normal use.
2. Description of the Prior Art
There are many applications which require monitoring the angular rotation of a shaft. For example, in the automotive environment the position or rate of change of the steering wheel is used to activate safety devices such as lighting directed into a turning lane, cancel turn signals upon completion of a turn, and provide an input for rear wheel steering in some of today's modern cars. A single turn potentiometer alone is, of course, unsuitable for monitoring the angular position of a steering wheel since a single turn potentiometer can only be used to sense the relative position of a shaft for only one revolution, and the steering wheel of most automobiles must be rotated more than one and typically more than two complete revolutions to execute a hard left or right turn. Common mechanisms used today to monitor the relative position of a shaft that rotates more than one revolution include various pulse counting circuitry arrangements and using multi-turn or gear driven potentiometer devices.
In the pulse counting circuitry schemes, incremental pulses generated by optical or magnetic pulse sensing circuitry are counted and are used to calculate relative position. These systems assume an initial starting or home position upon energizing. After activating the sensors, any movement of the rotary shaft under test is then translated as a position relative to the assumed starting point. A disadvantage of the pulse counting circuitry schemes is that the index or home position is redefined each time the sensor is re-activated after power has been removed from the system. If the sensors do not have a memory feature which stores the last known position or the position is changed during the power off mode, the home location established on re-activation will be in error. In more advanced pulse counting systems, error correction is often provided via a mathematical time based algorithm that averages the implied position readings from the sensor to statistically impute the normal index position. However, this type of correction method may not be satisfactory due to the time period required for the system to gain enough readings in order to redefine the index position. In addition, circumstances could prevail that the algorithm cannot address correctly.
Multi-turn or gear driven potentiometers have not proven to be satisfactory since, in order to mechanically perform their function, the devices tend to become bulky or unreliable due to their added complexity. Accuracy and resolution in the gear driven device is sacrificed because it must mechanically resolve multiple turns in less than one full potentiometer revolution.